It is indeed in December, there’s no official website specifically for the festival at the moment but the following site, though not Cuban, has been running for years and is very reliable: jazzcuba.com
It’s a site for people who want to visit the festival from the U.S but it does the job.

(And if you plan a trip to coincide, you might like to know that a new edition of the Rough Guide to Cuba is due out in November).